1. Field of the Invention
This invention is in the field of detectors, for example, lead detectors using x-ray fluorescence (XRF).
2. Description of the Prior Art
Many studies have shown that lead is a poison especially harmful to children. The Environmental Protection Agency and HUD have determined that lead paint in houses is a root cause of the poisoning of children and have recommended its elimination. Lead paint was in common use throughout most of the country until the 1960's; it was not completely phased off the market until the 1970's. Now States, including Massachusetts, have passed laws requiting its removal from residential dwellings. A central problem to a stronger effort to eradicate the lead paint hazard is the difficulty and expense of testing for the low lead concentrations considered dangerous.
The problem has been thoroughly discussed in publications from the National Institute of Standards and Technology. Two recent review publications from the NIST are particularly relevant. Methods for Measuring Lead Concentrations in Paint Films by M. E. McKnight, W. E. Byrd, W. E. Roberts and E. S. Lagergren, U.S. Department of Commerce, National Institute of Standards and Technology, 1989, NISTR 89 4209, gives is a comprehensive examination of the methods for measuring lead concentrations in paint films. Screening Procedures for Detecting Lead in Existing Paint Films by M. E. McKnight and W. E. Byrd, U.S. Department of Commerce, National Institute of Standards and Technology, 1990, NISTR 89-4044, is a literature survey of the screening procedures for detecting lead in existing paint films. Three general methods for finding lead paint on surfaces are available: chemical tests, absorption spectroscopy and x-ray fluorescence.
1) The chemical tests make use of the fact that the color of certain chemicals will change when exposed to lead. To use the technique, the inspector places the chemical on a chip of the paint, or places the chemical in a cut made through the paint on the wall. Chemical tests are inexpensive, a home-test kit can be purchased for less than 10 dollars. Chemical tests are not recommended by the various regulatory bodies, however, because of their many drawbacks: they are destructive of the surface, the tests are qualitative rather than quantitative, the tests cannot be easily repeated, and the tests do not result in a hard-copy output for the record.
2) Absorption spectroscopy is a quantitative, sensitive technique but requires that paint samples be taken to a laboratory to be analyzed by highly trained technicians using complex and costly spectrometers.
3) X-ray fluorescence is the preferred technique for field use. In principle, it obviates all of the drawbacks of the other two methods. The tests can be done with a portable instrument in a non-destructive, repeatable and quantitative manner so as to produce a validated, hard-copy output at the test point. X-ray fluorescence analysis, which is well-described in the literature, such as X-ray Fluorescence Spectrometry by Ron Jenkins, John Wiley & Sons publishers, 1988, makes use of the fact that one can determine the presence and amount of an element by measuring its characteristic x-rays emitted when the atoms of the material are excited by initiating photons. The XRF methods based on K x-ray detection make use of the fact that the innermost electrons of the lead atom, the so-called K-shell electrons, are bound to the nucleus with an energy of .about.88.005 keV. When a photon with energy greater than 88.005 keV strikes the lead atom, there is a probability that the K-electron will be ejected leaving a vacancy in the K-shell. That vacancy is quickly filled by one of the outer electrons, producing a spectrum of K x-rays unique to lead. The principal K x-ray lines are: K.sub..alpha..sbsb.1 =74.96 keV (most intense), K.sub..alpha..sbsb.2 =72.79 keV, K.sub..beta..sbsb.1 =84.922 keV and K.sub..beta..sbsb.2 =87.35 keV. X-ray-fluorescent (XRF) lead detectors on the market today use the 122 keV gamma rays from .sup.57 Co sources, with a half-life of 271.8 days, to photo-electrically excite the lead, and measure the lead K x-rays emitted from the sample surface, using an appropriate detector. Unfortunately, present x-ray fluorescent instruments are expensive, costing between $8,000 and $20,000, and the radioactive sources must be replaced, at least every year, at a cost between $1,500 and $3,500, depending on the strength needed. Since present instruments require a minimum of 20 seconds to make a single measurement of 1 mg/cm.sup.2 of lead, and three measurements are required for each test location, a complete lead paint analysis takes several hours of an inspector's time, and can cost more than all other home inspections combined.
The instruments on the market today have difficulty with false alarms, as emphasized by McKnight et al in NISTR 89 4209, due in part to the fact that the measured signal depends on the substratum under the paint. Thus, the count rate in the K x-ray window changes as the detector scans the walls of the room, even in the absence of lead. These substratum effects are well known confounders for present XRF portable paint analyzers.
The NISTR 89-4044 publication by McKnight and Byrd list six criteria for a lead screening test. "The procedures should be: 1) capable of detecting lead concentrations of 1 mg/cm.sup.2, 2) non-hazardous, 3) suitable for use as a nondestructive or minimally destructive field method, 4) suitable for use by non-technical personnel, 5) sufficiently reliable, precise and accurate and 6) rapid" The conclusion of the NISTR report (page 10) is that "Based upon the literature, no screening methods were found for detecting lead in paint that meet all the criteria.."
The present invention is aimed at developing a less expensive, more reliable lead paint detector that meet all six criteria, by utilizing the L x-rays of lead, together with the Compton scattered intensity, in a novel way.
The use of L x-rays to measure the lead in surfaces is well known but poorly regarded. The comprehensive NISTR 89-4209 report dismisses the method by citing only one reference, a paper by S .D. Rasberry, Investigation of Portable X-ray Fluorescence Analyzers for Determining Lead on Painted Surfaces, Applied Spectroscopy, 27, pp 102-108 (1973). Rasberry discusses L x-ray methods in only a few sentences on page 103 of the reference: "It is important to note that only analyzers which detect a K x-ray line of lead should be used in searching for lead in wall paint. The L x-ray lines are so low in energy that they are readily absorbed in any overlayers of non-lead paint, leading to probable false-negative results whenever several layers of paint are superimposed." Rasberry does, however, acknowledge the sensitivity of the technique in the next sentence of the reference: "Analyzers which detect L-lines are quite sensitive for lead in single layers of paint, but their use should be restricted to newly made and painted items such as toys or furniture." Rasberry's conclusions were not based on empirical data, yet have had considerable influence on the field; his conclusion is still cited by the experts almost twenty years later. In fact, as we will show, the L x-ray technique can be used to quantitatively determine low-levels of lead concentration under the 10 layers of TiO.sub.2 paint that Rasberry uses as his maximum test thickness. We also note that we have been unable to find any reference, such as Jenkins cited above, that makes use of the unique properties of the absorption of L x-rays that is our special discovery and which allows the concentration of heavy elements such as lead to be determined in the field, independent of the thickness and nature of the covering layers of non-lead paint.